feat: show platform prefix in bake logs

[121watts]

Summary

When running depot bake with multiple targets on different platforms, the logs were like provided no platform info to help you figure out which arch was doing what.

What was happening

Bake logs showed target names but not platforms:

[frontend 1/4] FROM alpine
[backend 1/4] FROM alpine

Even though frontend was building on amd64 and backend on arm64!

What happens now

Bake now tells BuildKit to show platform prefixes:

[frontend linux/amd64 1/4] FROM alpine
[backend linux/arm64 1/4] FROM alpine

Much easier to follow what's happening where. 🎯

Changes

• bake.go: Set DEPOT_FORCE_PLATFORM_PREFIX=true build arg for all bake targets
• build.go: Convert that build arg to force-platform-prefix frontend attribute

Test plan

• CLI builds successfully
• Existing bake tests pass
• Deploy with buildkit-private changes (https://github.com/depot/buildkit-private/pull/37)
• Run depot bake --progress=plain and verify platform prefixes show

Anything else?

This depends on https://github.com/depot/buildkit-private/pull/37 being deployed first — the CLI sends the attribute, but builders need to understand it.

Closes DEP-2793

[linear]

DEP-2793 bake logs are missing their platform prefix

[121watts]

Closing this in favor of a different approach.

The buildkit frontend modification won't work for builds using custom # syntax = ... directives, which bypass our frontend entirely.

Pivoting to a UI-based solution that infers platform from the build machine instead.